This invention concerns apparatus for determining the position of a pointer with respect to the surface of a tablet, such apparatus commonly being referred to as a digitizer. More specifically, this invention concerns digitizers wherein parallel conductors arrayed in a grid across a coordinate dimension are sequentially enabled to permit electrical induction to take place between the enabled conductor and a coil in the pointer, for producing a signal having a characteristic which can be detected when the conductor or conductors in nearest proximity to the pointer are enabled for stopping a clock or timer started at the enabling of a grid conductor corresponding to an origin, to determine a coordinate of the pointer position which is proportional to the elapsed time measured by the timer.
It is known in the art of digitizers to drive a grid of sequentially enabled conductors with a signal which is induced in a pointer, while continually monitoring the signal induced in a pointer, to detect a change in amplitude or phase indicative of the passing of the grid energizing signals swept along the conductors beneath the pointer. It is also known to energize the pointer coil and to monitor the output signals induced in sequentially energized grid conductors.
U.S. Pat. No. 4,088,842 to Ikedo for an Automatic Coordinate Determining Device discloses a digitizer, having a grid of scanned parallel sequential conductors, which depends on magnitude sensing to determine the coordinates of a pointer. It has also been found that highly accurate pointer determination is possible in a digitizer employing a scanned grid by detecting the change in phase of the voltage induced in the pointer or grid, depending on which of the pointer and grid is driven with an AC signal, as the scanning signal which enables the grid wires passes beneath the pointer. Examples of digitizer systems which depend on phase change detection to stop a counter or timer, started at the commencement of grid scanning by grid conductor enabling, are disclosed in U.S. Pat. No. 3,904,822 to Kamm et al for an Absolute Position Determining System Using Free Stylus, U.S. Pat. No. 4,080,515 to Anderson for Successively Electromagnetically Scanned X-Y Grid Conductors with a Digitizing System Utilizing a Free Cursor or Stylus, and U.S. Pat. No. 4,243,843, assigned to the assignee of the present invention by Rocheleau for a Coarse Position Digitizer. The foregoing patents disclose digitizer systems wherein the detection of a time of change of phase of the induced signal is performed by complex equipment requiring extensive use of digital circuitry and the processing of numerous signals, some of which must be derived by analog to digital conversion.
The above mentioned prior art phase detection digitizers are also limited with respect to the number of coordinate measurements which can be made over a period of time as the rate of grid scanning, that is, the rate at which the grid conductors are enabled is limited by the frequency of the grid scanning signal. The longer the tablet, the greater is the number of needed grid conductors and the longer is the time required to scan its full length in each dimension. The aforementioned Ikedo patent discusses provision for storing the address of the pointer and limiting future scans to an area less than the full dimension of the tablet.